Conventionally, there have been available onboard blind-spot camera image display apparatuses (blind corner monitors) that have a camera at the front end or the like of a vehicle so as to display, on a display device, images of the scenes to the right and left of the vehicle, thereby assisting the driver in judgment at intersections or the like where there are many blind spots. The driver can confirm the scenes to the right and left of the vehicle by viewing images on the monitor device so as to reduce the risk of overlooking approaching objects.
However, overlooking of and erroneous judgment about objects approaching from distant spots have still occurred, leaving a possibility of accidents occurring no matter how much the driver is paying attention to the monitor device. This is due to the following reasons.
An object approaching from a distant spot occupies only a small area in an image, making the object unnoticeable in the background, and the shapes of automobiles or motorcycles or the like cannot easily be recognized by drivers. Also, because these objects are approaching, they are moving only over a small distance in the images, making it difficult for drivers to recognize the movement. In view of this, a technique has been provided that detects approaching objects through image processing performed on camera-image data so as to emphasize the area displaying those approaching objects (for example, see Patent Document 1).
As one of the techniques for detecting moving objects in a moving picture, there are optical flow methods, which can roughly be categorized into a block matching method and a gradient method.
As described above, an object approaching from a distant spot moves only over a short distance in an image, and this distance is, in some cases, within one pixel. For example, for a blind corner monitor apparatus, about five frames are required to detect an approaching vehicle that is about fifty meters away from the camera under a particular lens setting, where a horizontal movement in one pixel is observed.
In such a case, when an optical flow is to be obtained by using a block matching method, the flow is calculated to be zero unless sub-pixel estimation is performed. Because of this, conventional moving object detection techniques have been insufficiently accurate in detecting objects at distant spots.
In the moving object judgment method in Patent Document 1 above, a gradient method, including a sub-pixel estimation function, is used for an optical flow of a distant spot, and thereby a flow of a movement within one pixel is calculated.
However, a gradient method uses a mathematical approximation under an assumption that luminance is consistent between two frames. This has resulted in the drawback that calculated values are often unstable.
Also, when sub-pixel estimation is performed, a non-zero flow is also calculated for a still area while a value of a flow of one pixel or smaller is calculated for a moving area, making it difficult to discriminate between a moving area and a still area.
Further, the technique of Patent Document 1 has a problem wherein information indicating whether a particular area in an image is a distant area or a near area is needed in order to discriminate between a moving area and a still area on the basis of a block matching method and a gradient method.
Further, a gradient method involves complex division processes. This slows the processing speed, and makes it difficult to mount this method on hardware. Also, a gradient method detects very small movements, causing the problem of picking up a lot of noise.
When a flow smaller than one pixel is to be calculated using a block matching method, the interval between two frames used for the flow calculation has to be longer than one time point. However, when the time interval between two frames to be used is simply expanded, a position change of objects moving at a high speed or of objects existing near the camera is made too great in images, and this causes a situation in which a point in a frame corresponding to a point in another frame is out of the search scope. Thereby, an erroneous flow is calculated, which has been problematic.
There has also been a problem that when a moving object is to be detected for each frame, more image frames have to be stored for the expanded intervals, requiring a great amount of memory, which has been problematic.    Patent Document 1: Japanese Laid-open Patent Publication No. 2007-172540